Aqueous polymer emulsions have hitherto been widely used in many applications such as adhesives for wood, paper, and plastics, paints, fiber processing agents, and paper processing agents. In particular, polyvinyl alcohol (which may hereinafter be abbreviated as “PVA”) is known to act as a protective colloid in emulsion polymerization of vinyl ester monomers typified by vinyl acetate. Aqueous vinyl ester emulsions resulting from emulsion polymerization employing a PVA as an emulsion polymerization dispersant are widely used in applications such as various adhesives for paper, woodworking, and plastics, various binders for impregnated paper and non-woven textiles, admixtures, placing joint materials, paints, paper processing, and fiber processing.
When a PVA is used as a dispersant for emulsion polymerization as described above, it is necessary to prevent coagulation of the emulsion during emulsion polymerization and allow good polymerization stability to be achieved. PVAs are water-soluble, and this may cause disadvantages such as a reduction in water resistance of a coating formed from an emulsion obtained using a PVA and a reduction in mechanical stability of the obtained emulsion. The properties such as the water resistance and mechanical stability are known to significantly depend on the PVA used in emulsion polymerization.
Under these circumstances, attempts such as crystallinity control of PVAs or functional group introduction into PVAs have been made to improve specific properties of PVAs, and accordingly various modified PVAs have been developed. There has been proposed an adhesive containing an aqueous emulsion and a polyfunctional isocyanate compound, the aqueous emulsion being obtained by emulsion polymerization of an ethylenically unsaturated monomer and/or a diene monomer using an ethylene-modified PVA as an emulsion polymerization dispersant. This adhesive has been reported to have high storage stability, bond strength, and water resistance (Patent Literature 1). As an emulsion obtained using a modified PVA there has been proposed an aqueous emulsion obtained using as an emulsion polymerization dispersant a modified PVA containing a specific amount of unsaturated monomer units having a silyl group and using as a dispersoid a polymer of one or more monomers selected from ethylenically unsaturated monomers. This aqueous emulsion has been reported to have high water resistance and viscosity stability (Patent Literature 2).
As stated above, emulsions are required to have good mechanical stability and be capable of forming a coating with good water resistance. Adhesives that can be applied at a high speed are also increasingly demanded in industrial production. When a conventional adhesive is applied at a high speed exceeding 500 m/min, there occurs an undesired phenomenon such as scattering of the adhesive which is so-called “splashing”. In addition, adhesives may be exposed to a harsh environment such as a hot or humid environment depending on the type of products in which the adhesives are used. Some conventional adhesives may, in such cases, experience a decrease in bond strength leading to separation, which means that they are unsatisfactory in terms of bond durability.
Under such circumstances, adhesives are required to be free from splashing during high-speed application, have high initial bond strength, and exhibit high bond durability at high temperature and high humidity.
However, the conventional emulsions and adhesives described above fail to meet all of the requirements, and an emulsion and an adhesive that meet these requirements are demanded.